Reactive power control has been widely deployed to enhance power quality and voltage profile, and to minimize loss, in an electric grid. Until now, the deployed solutions, such as capacitor banks owned by the utility operating the network, are either fixed or manually switched or automatically switched on a slow time-scale. Recently, there have been proposals to perform reactive power control automatically on a faster time-scale (seconds). One leading motivation is to utilize a certain type of inverters which generate or consume reactive power as a function of observed voltage. Such inverters may be found in variable power sources, e.g., solar, wind, and the like, to help correct the rapid voltage fluctuations on the distribution feeders of the electrical grid introduced by these same variable power sources.
One proposal to perform reactive power control faster and automatically is the recent Rule 21 draft from the California Public Utility Commission (CPUC). This proposal uses a closed-loop control philosophy where the voltage observed by the inverter determines the reactive power output (of the inverter), which in turn influences the voltage observed by the inverter (as well as voltages at other locations on the same feeder network).
While such a closed-loop control technique represents a direct attempt to solve the problem of voltage fluctuations on the various feeders of the electrical grid there are several potential problems which may be associated with this proposed method.
First, as with any closed feedback loop, there exists the possibility of instability. Specifically, in reaction to an observed voltage value V1, the inverter might output a reactive power value Q1, which causes the observed voltage to change to value V2, which forces the inverter to output a reactive power value Q2, which causes the voltage to change to value V3, and the like. There is no inherent guarantee that the sequence will converge to a stable equilibrium, and indeed various academic studies have shown the possibility of instability, where the sequence of voltages V1, V2, V3, etc., oscillate wildly. This may be especially worrisome in the case where the feeder of the electrical grid network contains a multitude of inverters, all performing its own feedback control while being oblivious of each other, but each of them affecting voltages seen by other inverters due to the inherent nature of reactive power flow. In a related phenomenon, even when the voltages ultimately settle down to equilibrium, the convergence time might be too slow (e.g. many seconds or even minutes).
Second, the inverters may be part of solar and wind generators which may be owned by customers of the electrical grid utility, e.g. residential roof-top solar panels, third-party owned wind/solar forms, and the like. While the utility operating the distribution feeder network has some control over what devices can be admitted (connected) to the network, such control is not perfect, especially given the expected diversity of hardware manufacturers of solar and wind generators. As a result, the utility cannot predict the precise behavior of the reactive power output of the inverters which are part of the solar and wind generators. A similar problem exists for commercial and industrial systems which utilize similar type inverters which generate or consume reactive power as a function of observed voltage. Thus, the utility cannot predict the equilibrium state of the network (assuming equilibrium will be reached), and cannot make confident guarantees on power quality, voltage level, and the like, which are necessary for regulatory compliance.
Third, by definition, a feedback loop based on observed voltage will only attempt to ameliorate voltage level problems. Traditionally, reactive power control is also used to improve power quality, minimize line loss, and the like, and these objectives remain important to the utility operating the electrical grid. Since the proposed control loop by CPUC does not use power quality measurements, e.g., as reactive current, power factor, and the like, as inputs, by definition it will have unpredictable effects on power quality and line loss. In some feeders and some operating conditions, the voltage-based feedback loop may happen to improve power quality and line loss, whereas in other feeders and other operating conditions, the voltage-based feedback loop may worsen power quality and line loss.
Clearly, from the perspective of a utility of an electrical grid, it would be ideal to be able to control the reactive power of inverters which generate or consume reactive power as a function of observed voltage—even if the inverters are not owned by the utility. Such control would need to be fast, stable, predictable, and precise. Then, the utility could employ any desired algorithm to determine the reactive power output levels in order to optimize any desired combination of voltage compliance, power quality, line loss, and the like.